National income refers to the total income earned by a country's citizens and businesses in a given period. It includes all income earned by citizens and companies both domestically and abroad, minus income earned by foreign entities within the country. Therefore, the answer to the question is not listed as an option. However, it is worth noting that Gross National Product at factor cost is often considered as the best measure of national income, as it includes income earned by the country's citizens both domestically and abroad, and it is adjusted for taxes and subsidies.